1. Field of the Invention
This invention relates to a method of and a system for reading a recording medium on which an original image including a predetermined periodic pattern is recorded and generating an image signal.
2. Description of the Related Art
There has been put into practice in various fields an image reproduction system which outputs a reproduction image through a printer or the like on the basis of original image data obtained by reading out an original image recorded on a recording medium (photographic film, paper or the like) by the use of an image scanner or the like.
When certain kinds of phosphor are exposed to a radiation (x-rays, α-rays, β-rays, electron beams, ultraviolet rays), they store a part of energy of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as “a stimulable phosphor”. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as “stimulated emission”. In the medical field, there has been known a radiation image recording/reproducing system in which a stimulable phosphor sheet (a sheet provided with a layer of the stimulable phosphor) is exposed to a radiation passing through an object such as a human body to have a radiation image of the object stored on the stimulable phosphor sheet, stimulating light beam such as a laser beam is caused to scan the stimulable phosphor sheet bearing thereon the radiation image of the object, the stimulated emission emitted from the stimulable phosphor sheet upon exposure to the stimulating light is photoelectrically detected, thereby obtaining a radiation image signal, and then a radiation image of the object is reproduced on the basis of the radiation image signal as a visible image on a recording medium such as a photographic film or a display such as a CRT. See, for instance, Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395, 55(1980)-163472, 56(1981)164645 and 55(1980)-116340.
When recording a radiation image of an object on a recording medium such as the stimulable phosphor sheet, a grid in which linear members of a radiation-untransmissible material (a material which is very low in permeability to the radiation) such as lead and linear members of a radiation-transmissible material (a material which is high in permeability to the radiation) such as aluminum or wood are alternately arranged at fine pitches of about 4/mm is sometimes interposed between the object and the stimulable phosphor sheet in order to prevent the stimulable phosphor sheet from being exposed to the radiation scattered by the object. By the use of such a grid, exposure of the stimulable phosphor sheet to the radiation scattered by the object is suppressed and the contrast of the radiation image of the object is enhanced. However, when a radiation image including therein an image of the grid is enlarged or reduced, aliasing is caused according to the rate of enlargement or reduction. Further, if the aliasing is superposed on the spatial frequency of, for instance, a grid image, fine moiré fringes are generated, which deteriorates quality of the reproduced image.
In view of these conditions, this applicant has proposed an image signal generating method of obtaining an image signal, on the basis of which an image having less moiré fringes and less aliasing can be reproduced as disclosed in Japanese Unexamined Patent Publication No. 3(1991)-114039. However, in the image signal generating method, since a filtering processing for removing spatial frequency components of the grid image is carried out on the image signal, harmonic components of the grid image cannot be removed and moiré fringes and/or aliasing is caused due to the harmonic components of the grid image. When an image including therein an image of the grid is enlarged or reduced, further aliasing and/or moiré fringes is caused to adversely affect diagnosis. Under these conditions, this applicant has proposed an image signal generating method in which an image signal free from the aforesaid aliasing or moiré fringes is obtained by reading out an image at a sampling frequency not lower than double of the spatial frequency of the grid image, carrying out a filtering processing on the image signal to remove therefrom harmonic components of the grid image and then re-sampling the resultant image signal at a desired sampling frequency as disclosed in Japanese Patent Application 2000-257668.
However the signal generating method is disadvantageous in that it takes a long time to sampling the original image at such a fine sampling frequency and at the same time, sampling the original image at such a fine sampling frequency results in a large amount of data, which increases time required to processing the image. Further, in order to remove all the harmonic components by the filtering processing, it is necessary for the filter to have a large size so that the filter has a sharp attenuation characteristic, which adds to the cost.
Further, in the medical images, since the spatial frequency components of a part of the image necessary for diagnosis are distributed in a relatively low frequency band, existence of aliasing in a relatively high frequency components hardly involves a problem.